1. Field of the Invention
The present invention relates to a method of manufacturing a thin-film semiconductor device which is formed on a substrate having an insulating surface of a glass substrate or the like. Also, the present invention relates to a method of manufacturing a semiconductor device (for example, a thin-film transistor) using a thin-film semiconductor device.
2. Description of the Related Art
In recent years, attention has been paid to a thin-film transistor as a semiconductor device using a thin-film semiconductor device. In particular, attention has been paid to a structure in which a thin-film transistor is mounted on a liquid-crystal electro-optic device. This is to constitute the thin-film transistor using a thin-film semiconductor device which is formed on a glass substrate constituting the liquid-crystal electro-optic unit. In this case, the thin-film transistor is disposed on each pixel electrode of the liquid-crystal electro-optic device and functions as a switching element that controls charges going in and out of the pixel electrode. Such a structure is called an active matrix liquid-crystal display unit and is capable of displaying an image with very high quality.
The active matrix liquid-crystal display unit, in general, has a structure in which an IC circuit constituting a peripheral drive circuit for driving a pixel thin-film transistor is disposed on a glass substrate through the COG technique, or the like. In other words, there is adopted a structure in which a large number of IC chips are disposed on the peripheral portion of a pixel region where a large number of pixels are arranged in the form of a matrix.
However, this structure is complicated in wiring from the IC chip and lower in productivity and reliability. Also, the existence of an external circuit such as the IC circuit makes a liquid-crystal panel thick. This problem leads to the lowering of the degree of the general purpose of the liquid-crystal panel.
As a structure that solves such a problem, there has been proposed a structure in which a drive circuit for driving the thin-film transistor disposed in the pixel region is also integrated with the liquid-crystal display unit (for example, refer to Japanese Patent Examined Publication No. Hei 2-61032).
This has a structure in which a pixel region and a peripheral drive circuit for driving a thin-film transistor in the pixel region are integrated with each other on one translucent substrate (glass substrate is generally used). This structure can be made simple and high in the degree of the general purpose.
The peripheral drive circuit is comprised of a shift register, an analog buffer circuit and other circuits that deal with an image signal. It is needless to say that those circuits are demanded to operate at the operation frequency of the image signal.
In order to solve the above difficulty, there has been adopted such a structure that a method of dealing with the image signal is devised, or circuits are arranged in parallel. However, those devices lead to such a situation that the operation method or the circuit structure is complicated, to thereby lower the practicability.
In order to solve this problem, the characteristic of an amorphous silicon film is enhanced. To achieve this, it is useful that the amorphous silicon film is crystallized into a crystalline silicon film. For a method of obtaining the crystalline silicon film, there has been known a method of subjecting an amorphous silicon film which has been formed through the plasma CVD technique or the decompression thermal CVD technique to a heat treatment.
However, in case of using a thin-film transistor in the active matrix liquid-crystal electro-optic device, there arises such a problem that a glass substrate need be utilized as a substrate from the viewpoint of economics. In order to crystallize the amorphous silicon by heating, a heat treatment must be conducted at a temperature of 600° C. or higher for several tens hours or longer.
However, even though a thin-film transistor is constituted using the crystalline silicon film obtained in the above manner, the transistor cannot operate at a frequency except for several MHz or lower. In other words, the image signal generally has a frequency band of several MHz to several tens MHz or higher, and the general thin-film transistor does not normally operate at a frequency except for about several MHz or lower even though the crystalline silicon film is used therefore.
The glass substrate is warped or deformed by heating at 600° C. or higher for several tens hours or longer. This becomes remarkable in particular when the glass substrate is increased in area. Since the liquid-crystal electro-optic device is required to has a structure in which liquid crystal is interposed and held between a pair of glass substrates which are laminated at an interval of several μm, the deformation of the glass substrate causes the nonuniformity of display and therefore is not preferable.
In order to eliminate the above problem, a quartz substrate or a specified glass substrate that withstands a heat treatment at a high temperature may be used as a substrate. However, the quartz substrate or the specified glass substrate that withstands a high temperature is expensive and therefore difficult to use from the viewpoint of the production costs.
Also, there has been known a technique in which an amorphous silicon film is crystallized by the irradiation of a laser beam. In case of utilizing the irradiation of a laser beam, a crystalline silicon film which is partially very excellent in crystalline property can be obtained, whereas there arise such problems that it is hard to obtain the uniformity of the effect of irradiation of a laser beam over the entire film, and also that even the crystalline silicon films as obtained are increased in dispersion every process (in other words, low in reproducibility).